Method For Simulation Of Flows Research Articles

Coupled volume of fluid and phase field method for direct numerical simulation of insoluble surfactant-laden interfacial flows and application to rising bubbles

Coupled volume of fluid and phase field method for direct numerical simulation of insoluble surfactant-laden interfacial flows and application to rising bubbles

Simulation of interior ballistic two-phase flow of storage propellant charge

ABSTRACT The launch safety evaluation of artillery considering the properties changes of propellant charge during storage has attracted much attention, and the determination of the interior ballistic process of storage propellant charge has been a bottleneck problem. Large scale firing test is difficult to implement due to its high risk and uncontrollability, and simulation is limited by the difficulty in describing the interaction of propellant parameters. In order to describe the multiple physical fields in the chamber and reveal the influence mechanism of storage on interior ballistic process, the theoretical analysis and physical simulation are combined in this paper. The fracture and the subsequent combustion of the storage propellant charge in the chamber environment were physically simulated, and then, the interior ballistic two-phase flow simulation method considering the fracture of storage propellant charge is proposed by coupling these performance parameters. Though describing the interior ballistic process variation with storage time, this paper provides a practical evaluation method for the interior ballistic performance of energetic materials.

A multiblock (MIB) finite element method for accurate and efficient blood flow simulation

We present an efficient and accurate immersed boundary (IB) finite element (FE) method for internal flow problems with complex geometries (e.g. blood flow in the vascular system). In this study, we use a voxelized flow domain (discretized with hexahedral and tetrahedral elements) instead of a box domain, which is frequently used in IB methods. We highlight the applicability and efficiency of the proposed method in generating the flow domain mesh and removing unnecessary portions of the domain, compared to the standard IB methods that rely on a full Cartesian mesh. Our approach is attractive for internal flow problems that use IB methods. The proposed method accurately satisfies the boundary conditions on the immersed object using velocity and pressure correction procedures. The velocity correction applies implicitly such that the velocity on the immersed boundary (Lagrangian points) interpolated from the corrected velocity values computed on the mesh nodes (Eulerian nodes) accurately satisfies the prescribed velocity boundary conditions. The proposed method utilizes the well-established incremental pressure correction scheme (IPCS) FE solver, and the boundary condition-enforced IB (BCE-IB) method to numerically solve the transient, incompressible Navier-Stokes (NS) flow equations. We verify the accuracy of our numerical method using the analytical solution for the Poiseuille flow in a cylinder, and the available experimental data (laser Doppler velocimetry) for the flow in a three-dimensional 90° angle tube bend. We further examine the accuracy and applicability of the proposed method by considering flow within complex geometries, such as blood flow in aneurysmal vessels and the aorta, flow configurations that would otherwise be difficult to solve by most IB methods. Our method offers high accuracy, as demonstrated by the verification examples, and high applicability, as demonstrated through the solution of blood flow within complex geometry. The proposed method is efficient, since it is as fast as the traditional finite element method used to solve the Navier–Stokes flow equations, with a small overhead (not more than 5%) due to the numerical solution of a linear system formulated for the IB method.

A discrete unified gas kinetic scheme with sparse velocity grid for rarefied gas flows

In this paper, a discrete unified gas kinetic scheme (DUGKS) with a sparse grid method applied in velocity space (DUGKS-SG) is proposed for simulating rarefied gas flows. The DUGKS-SG decomposes the computationally demanding problem into smaller and independent subproblems, thereby reducing the computational costs and exhibiting good parallelism. Several numerical tests, including the two-dimensional Riemann problem and the lid-driven microcavity flow, have been conducted to validate the performance of the DUGKS-SG. Comparisons with the original DUGKS and the Direct Simulation Monte Carlo (DSMC) method demonstrate that DUGKS-SG can provide satisfactory results with improved efficiency. Specifically, a maximum speedup of 9.486 for a 2D case with 7 CPU cores and 13.035 for a 3D case with 8 CPU cores can be achieved. These results suggest that the proposed DUGKS-SG can serve as an efficient numerical method for rarefied gas flow simulations.

Integrating discrete sub-grid filters with discretization-corrected particle strength exchange method for high Reynolds number flow simulations

Integrating discrete sub-grid filters with discretization-corrected particle strength exchange method for high Reynolds number flow simulations

Experiments and three-dimensional flow simulations on twin-screw pumps operated as control valves for energy recovery

This study explores the characteristics of twin-screw pumps when operated in turbine, i.e., reverse mode as an alternative to traditional control valves in piping systems for recovering energy. Experimental and flow simulation methods are employed. By an overset grid technique, unsteady three-dimensional (3D) flow simulations are conducted, ensuring high cell quality and high spatial resolution, particularly in wall proximity and within the gaps. To assess viscosity effects, pump and turbine mode characteristics are analyzed for water and oil. Compared to water, a pronounced reduction of the flow rate dependence on pressure difference is revealed for highly viscous oil. The gap flow in oil is significantly affected by the spindles’ rotational speed and direction, whereas water displays minimal influence. Analyzing the variation in gap sizes, a 50% reduction in the minimum flank gap width leads to a slight rise in volumetric efficiency with negligible overall efficiency effects. Our findings underscore that utilizing twin-screw pumps as turbines for high-viscosity fluids can potentially recover up to 50% of the energy instead of dissipating it by using conventional control valves.

Development of new numerical approach to the flow and segregation behaviors of fresh concrete considering rebar restraint and its application in reinforced lightweight aggregate concrete

Although there are many studies on flow simulation methods for fresh concrete, there are very few studies on casting simulation of reinforced concrete, and in these studies the reinforcing bars are treated only as static boundaries without their restraining effect on the concrete flowing between them. In this study, we first developed a new numerical method based on the MPS method for simulating the flow and segregation behaviors of fresh concrete during casting reinforced member, considering the restraint of reinforcing bars by applying a normal stress to the concrete. Fresh concrete was treated as the double-phase granular fluid that follows a non-linear rheological model, and was represented by matrix mortar, and coarse aggregate particles with actual sizes and random shapes. Then, as an application of this numerical method, the flow and segregation behaviors of lightweight aggregate concrete (LWAC) in an L-shaped box with rebars were numerically investigated in addition to measurement. The comparison of the numerical and experimental results shows that this numerical method allows the LCA volume fraction to be calculated within 3 % error in predicting the segregation behavior of LCA in reinforced LWAC, and improves the accuracy of the flow simulation of LWAC by about 1.5 times. Furthermore, this numerical method can double the accuracy of concrete flow prediction, compared to treating the reinforcement as a static boundary.

Shedding of Cavitation Clouds in an Orifice Nozzle

Focused on the unsteady property of a cavitating water jet issuing from an orifice nozzle in a submerged condition, this paper presents a fundamental investigation of the periodicity of cloud shedding and the mechanism of cavitation cloud formation and release by combining the use of high-speed camera observation and flow simulation methods. The pattern of cavitation cloud shedding is evaluated by analyzing sequence images from a high-speed camera, and the mechanism of cloud formation and release is further examined by comparing the results of flow visualization and numerical simulation. It is revealed that one pair of ring-like clouds consisting of a leading cloud and a subsequent cloud is successively shed downstream, and this process is periodically repeated. The leading cloud is principally split by a shear vortex flow along the nozzle exit wall, and the subsequent cloud is detached by a re-entrant jet generated while a fully extended cavity breaks off. The subsequent cavitation cloud catches the leading one, and they coalesce over the range of x/d≈1.8~2.5. Cavitation clouds shed downstream from the nozzle at two dominant frequencies. The Strouhal number of the leading cavitation cloud shedding varies from 0.21 to 0.29, corresponding to the injection pressure. The mass flow rate coefficient fluctuates within the range of 0.59~0.66 at the same frequency as the leading cloud shedding under the effect of cavitation.

A Novel Connection Element Method for Multiscale Numerical Simulation of Two-Phase Flow in Fractured Reservoirs

Summary This paper presents a novel approach to the numerical simulation of fractured reservoirs, called the connection element method (CEM), which differs from traditional grid-based methods. The reservoir computational domain is discretized into a series of nodes, and a system of connection elements is constructed based on the given connection lengths and angles. The pressure diffusion term is approximated using generalized finite difference theory. Meanwhile, the transmissibility and volume of the connection elements are determined, and pressure equations are solved discretely to obtain pressure at nodes to approximate the upstream flux along connection elements. Then, we solve the transport equation to obtain oil saturation profiles with low numerical diffusion, utilizing the discontinuous Galerkin (DG) method. Moreover, the flow path tracking algorithm is introduced to quantify the flow allocation factors between wells. In all, the pressure equation can be solved at a global coarse-scale point cloud and the saturation equation is calculated at a local fine-scale connection element. In other words, CEM is of multiscale characteristics relatively. Finally, several numerical examples are implemented to demonstrate that CEM can achieve a relatively better balance between computational accuracy and efficiency compared with embedded discrete fracture modeling (EDFM). Furthermore, CEM adopts flexible meshless nodes instead of grids with strong topology, making it more practical to handle complex reservoir geometry such as fractured reservoirs.

Dual Domain Decomposition Method for High-Resolution 3D Simulation of Groundwater Flow and Transport

The high-resolution 3D groundwater flow and transport simulation problem requires massive discrete linear systems to be solved, leading to significant computational time and memory requirements. The domain decomposition method is a promising technique that facilitates the parallelization of problems with minimal communication overhead by dividing the computation domain into multiple subdomains. However, directly utilizing a domain decomposition scheme to solve massive linear systems becomes impractical due to the bottleneck in algebraic operations required to coordinate the results of subdomains. In this paper, we propose a two-level domain decomposition method, named dual-domain decomposition, to efficiently solve the massive discrete linear systems in high-resolution 3D groundwater simulations. The first level of domain decomposition partitions the linear system problem into independent linear sub-problems across multiple subdomains, enabling parallel solutions with significantly reduced complexity. The second level introduces a domain decomposition preconditioner to solve the linear system, known as the Schur system, used to coordinate results from subdomains across their boundaries. This additional level of decomposition parallelizes the preconditioning of the Schur system, addressing the bottleneck of the Schur system solution while improving its convergence rates. The dual-domain decomposition method facilitates the partition and distribution of the computation to be solved into independent finely grained computational subdomains, substantially reducing both computational and memory complexity. We demonstrate the scalability of our proposed method through its application to a high-resolution 3D simulation of chromium contaminant transport in groundwater. Our results indicate that our method outperforms both the vanilla domain decomposition method and the algebraic multigrid preconditioned method in terms of runtime, achieving up to 8.617× and 5.515× speedups, respectively, in solving massive problems with approximately 108 million degrees of freedom. Therefore, we recommend its effectiveness and reliability for high-resolution 3D simulations of groundwater flow and transport.

Analysis of the flow field characteristics of a centrifugal blood pump in a heartbeat cycle

Blood pumps are medical devices that can temporarily or permanently replace the heart to perform the job of blood pumping. To study the internal flow field characteristics of the centrifugal blood pump, high-speed photography (HSP) images of the internal flow in the centrifugal blood pump were taken, and the internal flow field was numerically simulated. According to the heartbeat rule, the visualization experiment method and numerical simulation method for the internal flow of the centrifugal blood pump were developed. The internal flow rule of the centrifugal blood pump during a heartbeat cycle was revealed, as well as the influence of pump construction, flow rate, and other factors on the pump’s flow field. The results indicate that the flow rate and the rotor/stator interference of the blade and volute tongue cause two periodic changes in the pump pressure and velocity. One is the cycle of the heartbeat, and the other is the cycle of the blade’s rotation. The research results are of great significance in reducing the mechanical damage of blood, optimizing the structure of the pump and improving the stability of the centrifugal blood pump.

A particle-resolved direct numerical simulation method for the compressible gas flow and arbitrary shape solid moving with a uniform framework

Compressible particle-resolved direct numerical simulations (PR-DNS) are widely used in explosion-driven dispersion of particles simulations, multiphase turbulence modelling, and stage separation for two-stage-to-orbit vehicles. The direct forcing immersed boundary method (IBM) is a promising method and widely applied in low speed flow while there is few research regarding compressible flows. We developed a novel IBM to resolve supersonic and hypersonic gas flows interacting with irregularly shaped multi-body particle. The main innovation is that current method can solve the interaction of particles and high-speed fluids, particle translation and rotation, and collision among complex-shaped particles within a uniform framework. Specially, high conservation and computation consumption are strictly satisfied, which is critical for resolving the high speed compressible flow feature. To avoid the non-physical flow penetration around particle surface, an special iterative algorithm is specially derived to handle the coupling force between the gas and particles. The magnitude of the velocity difference error could be reduced by 6–8 orders compared to that of a previous method. Additionally, aerodynamic force integration was achieved using the momentum equation to ensure momentum conservation for two-phase coupling. A high-efficiency cell-type identification method for each step was proposed, and mapping among LPs and cells was used again to select the immersed cells. As for the collision force calculation, the complex shape of a particle was represented by a cloud of LPs and the mapping of LPs and cells was used to reduce the complexity of the algorithm for contact searching. The repetitive use of the mapping relationship could reduce the internal memory and improve the efficiency of the proposed algorithm. Moreover, various verification cases were conducted to evaluate the simulation performance of the proposed algorithm, including two- and three-dimensional moving and motionless particles with regular and complex shapes interacting with high-speed flow. Specifically, an experiment involving a shock passing through a sphere was designed and conducted to provide high-precision data. The corresponding results of the large-scale numerical simulation agree well with those obtained experimentally. The current method supports flow simulations at a particle-resolved scale in engineering.

A Fully Resolved Simulation Method for Flows with Finite Size Suspended Magnetic Particles

A Fully Resolved Simulation Method for Flows with Finite Size Suspended Magnetic Particles

Method for scalable and performant GPU-accelerated simulation of multiphase compressible flow

Multiphase compressible flows are often characterized by a broad range of space and time scales, entailing large grids and small time steps. Simulations of these flows on CPU-based clusters can thus take several wall-clock days. Offloading the compute kernels to GPUs appears attractive but is memory-bound for many finite-volume and -difference methods, damping speedups. Even when realized, GPU-based kernels lead to more intrusive communication and I/O times owing to lower computation costs. We present a strategy for GPU acceleration of multiphase compressible flow solvers that addresses these challenges and obtains large speedups at scale. We use OpenACC for directive-based offloading of all compute kernels while maintaining low-level control when needed. An established Fortran preprocessor and metaprogramming tool, Fypp, enables otherwise hidden compile-time optimizations. This strategy exposes compile-time optimizations and high memory reuse while retaining readable, maintainable, and compact code. Remote direct memory access realized via CUDA-aware MPI and GPUDirect reduces halo-exchange communication time. We implement this approach in the open-source solver MFC [1]. Metaprogramming results in an 8-times speedup of the most expensive kernels compared to a statically compiled program, reaching 46% of peak FLOPs on modern NVIDIA GPUs and high arithmetic intensity (about 10 FLOPs/byte). In representative simulations, a single NVIDIA A100 GPU is 7-times faster compared to an Intel Xeon Cascade Lake (6248) CPU die, or about 300-times faster compared to a single such CPU core. At the same time, near-ideal (97%) weak scaling is observed for at least 13824 GPUs on OLCF Summit. A strong scaling efficiency of 84% is retained for an 8-times increase in GPU count. Collective I/O, implemented via MPI3, helps ensure the negligible contribution of data transfers (<1% of the wall time for a typical, large simulation). Large many-GPU simulations of compressible (solid-)liquid-gas flows demonstrate the practical utility of this strategy.

Two‐particle debris flow simulation based on SPH

AbstractDebris flow is a highly destructive natural disaster, necessitating accurate simulation and prediction. Existing simulation methods tend to be overly simplified, neglecting the three‐dimensional complexity and multiphase fluid interactions, and they also lack comprehensive consideration of soil conditions. We propose a novel two‐particle debris flow simulation method based on smoothed particle hydrodynamics (SPH) for enhanced accuracy. Our method employs a sophisticated two‐particle model coupling debris flow dynamics with SPH to simulate fluid‐solid interaction effectively, which considers various soil factors, dividing terrain into variable and fixed areas, incorporating soil impact factors for realistic simulation. By dynamically updating positions and reconstructing surfaces, and employing GPU and hash lookup acceleration methods, we achieve accurate simulation with significantly efficiency. Experimental results validate the effectiveness of our method across different conditions, making it valuable for debris flow risk assessment in natural disaster management.

An edge-based interface tracking (EBIT) method for multiphase-flow simulation with surface tension

We present a novel Front-Tracking method, the Edge-Based Interface Tracking (EBIT) method for multiphase flow simulations. In the EBIT method, the markers are located on the grid edges and the interface can be reconstructed without storing the connectivity of the markers. This feature makes the process of marker addition or removal easier than in the traditional Front-Tracking method. The EBIT method also allows almost automatic parallelization due to the lack of explicit connectivity.In a previous journal article we have presented the kinematic part of the EBIT method, that includes the algorithms for piecewise linear reconstruction and advection of the interface. Here, we complete the presentation of the EBIT method and combine the kinematic algorithm with a Navier–Stokes solver. A circle fit is now implemented to improve the accuracy of mass conservation in the reconstruction phase. Furthermore, to identify the reference phase and to distinguish ambiguous topological configurations, we introduce a new feature: the Color Vertex. For the coupling with the Navier–Stokes equations, we first calculate volume fractions from the position of the markers and the Color Vertex, then viscosity and density fields from the computed volume fractions and finally surface tension stresses with the Height-Function method. In addition, an automatic topology change algorithm is implemented into the EBIT method, making it possible the simulation of more complex flows. The two-dimensional version of the EBIT method has been implemented in the free Basilisk platform, and validated with seven standard test cases: stagnation flow, translation with uniform velocity, single vortex, Zalesak's disk, capillary wave, Rayleigh-Taylor instability and rising bubble. The results are compared with those obtained with the Volume-of-Fluid (VOF) method already implemented in Basilisk.

Development of unsteady reduced-order methods for multi-row turbomachinery flow simulation based on the computational fluids laboratory three-dimensional solver

Development of unsteady reduced-order methods for multi-row turbomachinery flow simulation based on the computational fluids laboratory three-dimensional solver

Enhanced parallel computation for time-fractional fluid dynamics: A fast time-stepping method with Newton-Krylov-Schwarz solver

This paper presents a sum-of-exponentials domain decomposition method for the numerical simulation of two-dimensional unsteady fluid flow and heat transfer using a time-fractional fluid model. We employ a fast time-stepping approach to discretize the time-fractional derivatives, followed by the application of a parallel Newton-Krylov-Schwarz algorithm to solve the resulting discrete nonlinear system. The numerical experiments demonstrate a superior performance of the fast time-stepping method compared to the traditional L1 scheme, particularly in the context of parallel computation, as it substantially reduces the computational complexity and memory usage. The algorithm exhibits robust performance across a wide range of model parameters and solver settings. Notably, it achieves 60% parallel efficiency with the use of 768 processor cores. This study underscores the efficacy of parallel processing as a potent computational strategy for addressing the challenges in solving time-fractional partial differential equations.

Multi-dimensional flow simulation analysis and kriging-based optimization study on thermodynamic process mechanism of pulse tube refrigerator with displacer

Pulse tube refrigerators with active displacers have become increasingly popular due to superior phase shift capability and avoidance of dissipation losses. However, mechanisms of thermodynamic processes within refrigerators due to variations in parameters related to critical structures and operations are not yet clear, making it difficult to optimize the comprehensive performance. In this paper, multi-dimensional flow simulation method instead of one-dimensional simulation method with poor computational accuracy is used to analyze the influence of five key parameters on performance of pulse tube refrigerators with active displacers. An intelligent optimization algorithm combined with efficient Kriging surrogate models is developed for single-objective and multi-objective optimization. Results demonstrate that the mean errors of the Kriging models for cooling capacity, total input power and performance coefficient are 1.81%, 3.94% and 3.37%. The single-objective optimization produces a maximum cooling capacity of 7.29 W and performance coefficient of 7.38%. The combined optimal solution delivers 6.65W in cooling capacity and 7.02% in performance coefficient, obtaining improvements of 150.94% and 75.94%. This study reveals the influence mechanism of key parameters on cooling performance of pulse tube refrigerators with displacers and establishes an efficient global optimization method, which is of great significance for the development of advanced refrigeration technology.

Impact of Biomass Fuel Feeding Ratio in Co-firing Circulating Fluidized Bed Boiler: A Computational Fluid Dynamics Study

Nowadays, our global population is on a notable rise, coupled with an annual surge in energy consumption. The prevailing reliance on fossil fuels, especially in electricity generation, has significantly contributed to environmental pollution and exacerbated global warming. The circulating fluidized bed, distinguished for its continuous operation and effective heat transfer in the combustion chamber, emerges as a prominent boiler type. Furthermore, the use of biomass fuel, recognized for its renewable and environmentally friendly characteristics, presents an attractive option. Hence, exploring a co-firing system incorporating both coal and biomass as fuel feeds for the boiler holds promise, necessitating optimization for efficient energy production and reduced gas emissions. This study employs computational fluid dynamics to simulate the intricate interactions of solid fuel and flue gas reactions within the boiler, utilizing the two-fluid method for multiphase flow simulation. The circulating fluidized bed boiler in focus employs subbituminous coal, woodchips as biomass sources, and sand as the bed material. Model validation against operational data, including bed temperature, flue gas velocity outlet, and carbon dioxide mass fraction, indicates minimal deviation. Examination of the biomass ratio's impact on fuel feed reveals a reduction in sulfur dioxide emissions with an increasing biomass ratio, attributed to the lower sulfur content in woodchips compared to coal. However, a heightened woodchip blending ratio results in diminished boiler efficiency due to the altered heating value of the mixed solid fuel. The optimized biomass-to-coal ratio in fuel feeding is determined as 59.15%, achieving a maximized boiler efficiency of 82.84% and minimized pollution gas emissions of sulfur oxide and nitrogen oxide in accordance with industrial standards.